New “provocative” research has predicted that herd immunity to COVID-19 could be achieved with less people being infected than previously estimated.
Epidemiologists have been working on the assumption that herd immunity will require 60 per cent of the population to be infected — from contact with a sick person or, eventually, by a vaccine.
According to mathematical modelling from the University of Nottingham and University of Stockholm, “when differences in age and social activity are incorporated in the model, the herd immunity level reduces from 60 per cent to 43 per cent.”
The researchers are at pains to advise that this figure of 43 per cent “should be interpreted as an illustration rather than an exact value or even a best estimate.”
While their basic reasoning appears sound, further modelling is required to get a more precise estimate.
Even so, they conclude that the findings are powerful enough to “have potential consequences for the current COVID-19 pandemic and the release of lockdown and suggests that individual variation (e.g. in activity level) is an important feature to include in models that guide policy.”
So how did they reach their findings?
Gaining herd immunity against COVID-19 is the holy grail. It means we will get our lives back to the point where social distancing and other preventative measures are no longer necessary. The economy and social order can begin to recover.
To get there requires a certain percentage of the population to become immune to the virus – from ether direct exposure and subsequent illness or from vaccination – “and the chain of transmission is broken,” as the researchers put it.
That magical percentage is an estimate based on what’s known as the reproductive number. This is the number of cases, on average, that an infected person will cause during their infection period. For SARS-CoV-2 it’s said to be between two and three.
Scientists have been operating on an assumption that herd immunity will be reached when 60 per cent of the population is immune.
But as the researchers note: “The figure of 60 per cent assumes that each individual in the population is equally likely to be vaccinated, and hence immune.
“However, that is not the case if immunity arises as a result of disease spreading in a population consisting of people with many different behaviours.”
In a prepared statement, one of the researchers, Professor Frank Ball from the University of Nottingham, explained:
“By taking this new mathematical approach to estimating the level for herd immunity to be achieved we found it could potentially be reduced to 43 per cent and that this reduction is mainly due to activity level rather than age structure.
“The more socially active individuals are then the more likely they are to get infected than less socially active ones, and they are also more likely to infect people if they become infected. Consequently, the herd immunity level is lower when immunity is caused by disease spreading than when immunity comes from vaccination.”
This is awkward: if we allow people to act irresponsibly, ditch social distancing, and spread the infection, immunity will be reached sooner than if we relied on a vaccine.
Sweden took such an approach and, with too many dead, has since regretted it. But it doesn’t mean the research isn’t practically useful.
Should the research have been published?
An editorial blog from senior editors at Science reveals that they “discussed whether it was in the public interest to publish the findings.”
While the paper the article received support from members the Board of Reviewing Editors and experts who provided peer review, “we were concerned that forces that want to downplay the severity of the pandemic as well as the need for social distancing would seize on the results to suggest that the situation was less urgent.”
However, Science decided that the benefit of providing the model to the scientific community was worthwhile.
“The effects of many variabilities on infection spread, including age, genetics, and past exposures to other vaccines and viruses, are beginning to emerge for SARS-CoV-2,” the editors wrote.
“Together with behavior, these factors will affect the degree to which different populations are susceptible to infection, and understanding population heterogeneity may guide vaccination strategies.”
But they caution that there is no data to suggest that any country “is close to achieving herd immunity.”
Continuing non-pharmaceutical interventions around the world, they write, “is still of great importance.